The ultimate goal of this research is the measurement of left ventricular efficiency as (work)/(work plus heat). Experiments will be done on anesthetized dogs chronically instrumented with an electromagnetic flowmeter on the ascending aorta and a left atrial catheter for microsphere injection. Thermistor catheters and manometers will be placed at the time of the experiment. Myocardial heat dissipation rate via the coronary blood flow will be obtained from the temperature difference between aortic and coronary sinus blood together with left ventricular coronary flow from microsphere determinations. In order to calculate total left ventricular heat dissipation rate, fractional distribution of myocardial heat dissipation between coronary blood and other routes will be determined from a heat pulse technique. A known quantity of heat will be injected into the coronary artery together with a known quantity of dye. The difference between the fraction of heat recovered in the coronary sinus blood and the fraction of recovered dye is a measure of the percentage of heat dissipated by the coronary blood flow. It is assumed that this percentage figure is also applicable to the dissipation of heat generated within the myocardium. Thus, total myocardial heat dissipation may be calculated. Efficiency will then be studied under different cardiovascular conditions using up to six isotope labels for the microsphere determinations.